This study will investigate mechanisms whereby the human pregnancy survives despite its similarity to a reciprocal allograft relationship. The major hypotheses to be explored in order to better understand the immunologic relationship of mother and fetus are: (1) placental syntrophoblast is an antigenically neutral zone that provides a barrier to maternal rejection, (2) cellular immunity of maternal lymphocytes is blocked by humoral factors in the placental bed, and (3) the developing human fetus possesses considerable immunocompetence which affords protection against transplacental passage of foreign antigens.